Salt is the only must.
Agreed, pepper is completely optional…There was a thing done about why we view pepper as essential as salt when it is not. Its a spice, not a seasoning.
Anchovy paste (western cuisine) or alternatively fish sauce (eastern cuisine) is not about season or spice, it is about flavor. It is always a great way to punch up umami. I use it regularly in most sauces that need depth or have high natural acidity.
any vinegar will give you great punch and brightness in a dish or sauce, balsamic is interesting and incredibly versatile as it can give brightness or sweetness depending on the type of BV you employ.
I used to be all about tons of garlic, but have ramped it down in recent years. Shallots are always a staple, as they have the best onion flavor without overpowering and do not change their flavor profile when cooked. Yellow or Vidalia onions turn to pure sugar when cooked, shallots do not. And also agreed on the tomato part…I use tomato paste often in small amounts and brown/toast it like you do any other spices. It brings amazing acidity and sweetness.
The other essential items for my spice cabinet are: Bay Leaves (I but fresh and dry them myself), vanilla beans, lavender, chili powder and assorted dried chilis, fennel seeds, coriander seeds, cumin and saffron. I also have an assortment of pastes and sauces for various Asian and Spanish dishes.